4. Interphase = G1, S, G2
Interphase is when the cell grows,
and the organelles double prior to
the actual splitting of the nucleus.
93% of a cell’s life is spent in
Interphase has three parts
Growth 1 (G1)
Growth 2 (G2)
5. G1, S, G2
G1 is when organelles double.
Remember each new cell needs a
complete set of organelles.
S when DNA is replicated.
Each cell needs a complete and
identical set of DNA
G2 Proteins needed for Mitosis are
The process by which the cell nucleus divides
into two identical cell nuclei.
In some Human cells interphases lasts 15.3
hours, while mitosis lasts only .7 hours.
Occurs in a series of steps
Must duplicate and separate during
Structures of the tightly packaged
DNA is tangled up into a substance
The chromatin is packaged on the
Chromosomes now called chromatids because
they doubled to form short thick rods which pair
up and line up in the center of the nucleus.
A centromere connects the two halves of the
Spindle fibers begin to form.
Spindle fiber – a fibrous structure from the
cytoplasm which forms to the centriole.
Centrioles move to opposite sides of the cell.
The nuclear membrane breaks down.
Centromeres of the chromatid pairs line up in
the middle of the cell.
Metaphase plate- location where the
centromeres line up in the center of the cell.
By the end of metaphase each chromatid has
attached to spindle fibers.
When the chromosomes reach
opposite sides of the cell the
spindle fibers break up.
The nuclear membrane begins to
A furrow begins to develop
between the two sets of
Diploid (2n) - A cell with two of each kind of
One chromosome from each parent.
If two body cells were to combine nuclei, the
number of chromosomes would double.
In order for sexual reproduction to occur, each
cell involved must reduce its chromosome
number by half.
Haploid (n)- A cell with one of each kind of
21. Haploid cells
Haploid cells are called gametes
Gametes are either sperm or eggs
Organism diploid gamete
Human 46 23
Pea 14 7
Fruit fly 8 4
Dog 78 39
22. Homologous chromosomes
Are paired chromosomes with genes for the
same trait arranged in the same order.
Ex. Eye color, hair color, height, one may
code for blue, blonde, tall, its homolog may
code for brown, blonde, short
Homologous chromosomes may have
different alleles on them
Allele- gene form for each variation of a trait
of an organism.
Meiosis is the process of cell division in which
gametes are formed and the number of
chromosomes is halved. So that sexual
reproduction and zygote formation can occur.
Zygote- Fertilized egg which has a diploid
number of chromosomes.
24. Stages of Meiosis
Each chromosome consists of 2 identical sister
Each Pair of homologous chromosomes come
together to form a tetrad.
Tetrad- 2 homologous chromosomes come
together and the 4 chromatids overlap.
25. Crossing over
Tetrads are so tight that non-sister chromatids
from the homologous pair actually exchange
Crossing over- The exchange of genetic
material by non-sister chromatids during late
prophase I of meiosis.
Results in a new combination of alleles
26. Metaphase I
Homologous chromosomes line up together in
* In mitosis homologous chromosomes line up in
the middle independently of each other.
27. Anaphase I
Spindle fibers attach to the centromeres of
Homologous chromosomes separate and
move to opposite ends of the cell.
Centromeres DO NOT split like they do in
Now each cell will get one chromosome from
each homologous pair.
28. Telophase I
Spindle fibers break down
Another cell division is needed because the
number of chromosomes has not been reduced
After telophase I there maybe a short
interphase, but not always. It is important to
note that if a cell does have a second
interphase, there is No replication of
30. Meiosis II
Is basically just like mitosis, but remember the
chromosomes did not duplicate in interphase II.
Chromosomes begin to line up in the middle of the
Spindle fibers begin to form
Chromosomes line up on the metaphase plate
31. Meiosis II
Sister chromatids separate and move to opposite
sides of the cell
Spindle fibers disappear
Cytoplasm divides into two.
The number of chromosomes in each daughter
cell has now been reduced by half.